The present invention relates to a method, an arrangement and the use of an arrangement for tuneable add/drop multiplexing and for wavelength selective switching.
There are known to the art a number of different methods for further enhancing the capacity of an optical transmission line (point-to-point connection) or in an optical network (multipoint-to-multipoint connection). One method involves the use of a so-called wavelength division multiplexing (WDM) technique for improving the extent to which available bandwidths are utilised on an optical fibre in the optical network, where the information is multiplexed with the aid of an optical wavelength multiplexer. The wavelength can also be used as an information address in an optical network. Enhancement of the flexibility of the network necessitates the presence of devices that are able to reroute traffic in the optical network. Such traffic rerouting devices, or arrangements, are also suitable for using the network in the most effective manner possible, and in the case of a network crash or gilch.
In the case of bus and ring networks for instance, add/drop multiplexers are central to effective communication with the network. It is important that the add/drop multiplexer can be tuned (change add/drop wavelength), when a node wishes to change communication channels. Swedish Patent Application SE 9700865-0 describes a wavelength selective 1-N switch and multi-add/drop with fixed add/drop wavelength channels.
This Swedish patent application also describes an arrangement which does not provide tuneable (selectable) add/drop multiplexing.
A number of different, known methods can be used to enhance the capacity of an optical transmission system.
In wavelength multiplexing, for instance, transmission channels are respectively multiplexed and demultiplexed to and from an information flow on different carrier wavelengths. This multiplexing and demultiplexing process requires the presence of optical wavelength selective devices. In bus and ring networks for instance, add/drop multiplexers are central to effective communication with the network. If a node wishes to change a communication channel, it is important to be able to tune the add/drop multiplexer (change add/drop wavelength).
One problem in this regard is that a known tuneable add/drop multiplexer may be consistent with high channel-dependent losses, crosstalk problems, and a high price.
The present invention addresses the aforesaid problem with a tuneable add/drop multiplexer that includes at least one MMI-waveguide, at least N-number of Michelson waveguides, where Nxe2x89xa74, at least one Bragg grating for each Michelson waveguide, and at least one phase control element in at least Nxe2x88x921 number of Michelson waveguides. The Michelson waveguides include said phase control elements and said Bragg gratings and are arranged for at least one MMI-waveguide. Each Michelson waveguide can be provided with a broadband reflection grating.
In a preferred embodiment, the broadband reflection gratings are arranged at the end of each Michelson waveguide.
In another embodiment of the inventive tuneable add/drop multiplexer, there is included at least one MMI-waveguide which is provided on a first side with at least four access waveguides and on a second side with at least four Michelson waveguides, one N-channel (de)multiplexer for each said Michelson waveguide, one reflection section per Michelson waveguide, wherein said reflection section includes N-number of Michelson waveguides for each said N-channel (de)multiplexer, wherein each such Michelson waveguide includes at least one Bragg grating, and wherein at least N-number of Michelson waveguides include at least one phase control element.
In another embodiment, the inventive tuneable add/drop multiplexer includes at least two MMI-waveguides, at least M-number of Michelson waveguides per MMI-waveguide, where Mxe2x89xa73, at least one Bragg grating per Michelson waveguide, and at least one phase control element in at least Mxe2x88x921 number of Michelson waveguides per MMI-waveguide. The Michelson waveguide includes said phase control element and said Bragg grating and is provided on a second side of the MMI-waveguides. An access waveguide provided on a first side of a first MMI-waveguide and an access waveguide provided on a first side of a second MMI-waveguide are mutually connected via a connection waveguide.
In a preferred embodiment of the aforesaid inventive, tuneable add/drop multiplexer, there is arranged an 1xc3x97N switch for each Michelson waveguide in connection with the second side of the MMI-waveguides, and a reflection section that includes said Bragg grating and said phase control element is provided for each such switch.
The MMI-waveguides are preferably a 3xc3x973 type waveguide. The tuneable add/drop multiplexer may also include a broadband reflection grating for each Michelson waveguide. These gratings are preferably arranged at the end of each Michelson waveguide.
In a further embodiment of the inventive tuneable add/drop multiplexer, said M-number of Michelson waveguides, where Mxe2x89xa73, are arranged between a second side of the first MMI-waveguide and a second side of the second MMI-waveguide. Each Michelson waveguide includes at least two Bragg gratings, and at least Mxe2x88x921 numbers of Michelson waveguides include at least three phase control elements. The components thus function as an MMIMZI (Multi Mode Interference Mach-Zehnder Interferometer) for transmitted channels.
According to yet another embodiment of the inventive add/drop multiplexer, the multiplexer includes a third and a fourth MMI-waveguide. At least M-number of Michelson waveguides, where Mxe2x89xa73, are arranged between the third and the fourth MMI-waveguide. Each Michelson waveguide includes at least two Bragg gratings, and at least Mxe2x88x921 number of Michelson waveguides include at least three phase control elements. An access waveguide provided on the third or on the fourth MMI-waveguide via a connecting waveguide, and an access waveguide provided on the third MMI-waveguide is coupled to an access waveguide on the fourth MMI-waveguide via a connecting waveguide.
The third and the fourth MMI-waveguides are preferably of the 3xc3x973 type.
The invention also relates to a first method for add/drop multiplexing optical waveguide channels in an optical network. Optical wavelength channels are excited into an access waveguide arranged on a first side of an MMI-waveguide. No optical add-wavelength channel or at least one optical add-wavelength channel is excited into a second access waveguide arranged on the first side of the MMI-waveguide. The optical wavelength channels, including the add-wavelength channel, are transmitted through said first MMI-waveguide and are imaged on at least one Michelson waveguide arranged on the opposite side relative to said access waveguide. The optical wavelength channels, including the add-wavelength channel, are transmitted through the Michelson waveguides. The phase of no optical wavelength channel or at least one of the optical wavelength channels is changed by any of the phase control elements in any of the Michelson waveguides.
At least one optical wavelength channel is reflected by a Bragg grating section arranged in the Michelson waveguide. No or at least one wavelength channel is dropped to a third access waveguide arranged on the first side of the MMI-waveguide. No or at least one wavelength channel is transmitted out through a fourth access waveguide arranged on the first side of the MMI-waveguide.
Those wavelength channels that have not been reflected by a Bragg grating can be reflected by a broadband reflection grating arranged in each of the Michelson waveguides.
The invention also relates to a second method for tuneable add/drop multiplexing of optical wavelength channels in an optical network. Optical wavelength channels are excited into a first access waveguide arranged on a first side of a first MMI-waveguide. The optical wavelength channels are transmitted through said first MMI-waveguide and imaged on at least one Michelson waveguide arranged on an opposite side in relation to said access waveguide. The optical wavelength channels are transmitted through the Michelson waveguides. The phase of none or at least one of the optical wavelength channels is changed by any of the phase control elements arranged in each Michelson waveguides. At least one optical wavelength channel is reflected by at least one Bragg grating section arranged in the Michelson waveguides. None or at least one wavelength channel is dropped to a second access waveguide arranged on the first side of the first MMI-waveguide. At least one wavelength channel is transmitted out through a third access waveguide arranged on the first side of the first MMI-waveguide. Said wavelength channel is transmitted through a connecting waveguide arranged between the first and the second MMI-waveguide. Said wavelength channel(s) is/are transmitted through said second MMI-waveguide and imaged on at least one Michelson waveguide arranged on the opposite side relative to said access waveguide.
The phase of no wavelength channel or at least one optical wavelength channel is changed by any phase control element arranged in any of the Michelson waveguides. At least one optical wavelength channel is reflected by a Bragg grating section arranged in the Michelson waveguides. None or at least one add-wavelength channel is excited into a second access waveguide arranged on the first side of the second MMI-waveguide. None or at least one wavelength channel is transmitted out through a third access waveguide arranged on the first side of the second MMI-waveguide.
Those wavelength channels that have not been reflected by a Bragg grating can be reflected by at least one broadband reflection grating arranged in each Michelson waveguide.
The wavelength channel or wavelength channels that is/are transmitted out through the third access waveguide arranged on the first side of the second MMI-waveguide can be excited in via a connecting waveguide in a first access waveguide arranged on a first side of a third MMI-waveguide. The optical wavelength channels are transmitted through said third MMI-waveguide and imaged on at least one Michelson waveguide arranged on an opposite side relative to said access waveguide. The optical wavelength channels are transmitted through the Michelson waveguides. The phase of at least one of the optical wavelength channels is changed by a phase control element arranged in a Michelson waveguide. At least one optical wavelength channel is reflected by at least one Bragg grating section arranged in the Michelson waveguides. No wavelength channel or at least one wavelength channel is dropped to a second access waveguide arranged on the first side of the third MMI-waveguide. At least one wavelength channel is transmitted out through a third access waveguide arranged on the first side of the third MMI-waveguide. Said wavelength channel is transmitted through a connecting waveguide arranged between an access waveguide on the third MMI-waveguide and an access waveguide arranged on the fourth MMI-waveguide. Said wavelength channel is transmitted through said fourth MMI-waveguide and imaged on at least one Michelson waveguide arranged on an opposite side relative to said access waveguide. The phase of no optical wavelength channel or at least one optical wavelength channel is changed by any phase control element arranged in any of the Michelson waveguides. At least one optical wavelength channel is reflected by at least one Bragg grating section arranged in the Michelson waveguides. No add-wavelength channel or at least one add-wavelength channel is excited into a second access waveguide arranged on the first side of the fourth MMI-waveguide. No wavelength channel or at least one wavelength channel is transmitted out through a third access waveguide arranged on the first side of the fourth MMI-waveguide.
The aforesaid MMI (Multi Mode Interference) structure is used for splitting and as a phase-dependent combiner of light. The intensity distribution of light at the inputs of an MMI structure is imaged on all outputs of the MMI structure, provided that its length has been correctly chosen. A more profound theory behind this is found in L. B. Soldano and E. C. M. Pennings, xe2x80x9cOptical Multi Mode Interference Devices Based on Self Imaging: Principles and Applicationxe2x80x9d, J. Lightwave Technology, Vol. 13(4), pp. 615-627, 1995.
Bragg gratings are used to filter light. The grating allows light of certain wavelengths to pass through while reflecting light of other wavelengths. Bragg gratings can be said to form some kind of wavelength selective mirror. A more basic theory can be read from Phase-shifted Fiber Gratings and their Application for Wavelength Demultiplexing, IEEE Photon. Tech. Lett., Vol. 6(8), pp. 995-997, 1994. In, for instance, SiO2/Si, a periodic material index is created in the waveguide, by illuminating said waveguide periodically with UV light.
The aforesaid phase control elements are required for certain switching functions and for correcting process imperfections. Several types of phase control elements are known. However, a basic feature of these elements is that the optical wavelength is influenced by an applied external signal (voltage, current, light or heat). Normally, there is used a so-called thermooptical element, that is to say the refractive index and therewith the wavelength is influenced with the aid of heat (a temperature change results in a change of the refractive index.
The invention also includes the use of a wavelength selective switch and a tuneable add/drop multiplexer. The wavelength selective switch includes at least one MMI-waveguide, at least four Mach-Zehnder waveguides, at least one Bragg grating, and at least one phase control element with each Mach-Zehnder waveguide. Where said Mach-Zehnder waveguide includes said phase control element and said Bragg grating and is provided for at least one MMI-waveguide.
The object of the present invention is to provide a tuneable add/drop multiplexer that can achieve lower losses, channel-independent losses, and less crosstalk problems than are experienced with available, tuneable add/drop multiplexers, and also to obtain a wavelength selective switch with dimensions Mxc3x97N, where M and N are positive integers.
The invention will now be described in more detail with reference to preferred embodiments thereof and also with reference to the accompanying drawings.